The graphic novel, and the film that followed is about the historical Battle of Thermopylae.  In that battle, the Spartans were tasked with slowing down the Persian ground forces until the Greek navy could defeat the Persians on the seas. Most of the information that historians have about Thermopylae is presented in the accounts of the Roman historian Herodotus. Herodotus had a habit of blending history with myth, so a lot of the story seems exaggerated.  While the battle did take place, and the Spartans did mount a very brave defense to slow down the Persians, it is unlikely that it was done with only three hundred soldiers.  So, to answer the original question, the novel is somewhat true, but likely an exaggeration of an already exaggerated battle in history.